The goal of this project is exploratory research into localization of distributed sources of neural activity from joint MEG/EEG measurements, using a recently developed methodology for the inverse problem (Local Basis Expansions or LBEX). Neural dynamics in the 10-100 ms timescale underlie a broad spectrum of cognitive function relevant to the study of mental disorders. EEG and MEG are the only practical noninvasive techniques with this time resolution, but are limited in spatial resolution due to the ill posed inverse problem relating sensor measurements to underlying sources. EEG is more widely used, but also suffers from uncertainties about head conductivity profiles. The MEG inverse problem has proven to be comparatively more tractable. However, MEG is substantially more expensive, and it is desirable to combine the two techniques, so that EEG sources can be better pinpointed using simultaneous MEG measurements. Another reason to combine the two methods is the complementary nature of the inverse problems: EEG silent sources can be MEG active, and vice versa. Thanks to recent instrumental advances, simultaneous measurements are now routinely possible, but the joint localization problem has not yet been fully studied. We have developed a methodology for MEG source localization (LBEX), which permits a systematic analysis of resolution limits, through a mathematical treatment of the uncertainty principle governing the inverse problem. In this proposal, we propose to explore extensions of the LBEX method to joint MEG/EEG source localization. Specific aims include (i) a theoretical analysis of the joint localization problem in an idealized spherical model, (ii) evaluation of the performance of the technique in numerical simulations with a realistic head model, and (iii) application of the joint localization technique to simultaneous MEG/EEG recordings obtained through an experimental collaboration. Successful completion of the research program will establish the utility of our algorithmic framework, and will also evaluate the gain obtained from simultaneous MEG/EEG localization. Once the methodology is validated by the exploratory research, we intend in the future to encode the results into user friendly software which will enable widespread usage of joint EEG/MEG localization.